Birefringent solid film of evaporated and deposited isophthalic acid



350-406 SR if 556 fitAl-(UH K'UUM Aug. 24, 1948. E. R. BLOUT 2,447,792

' BIREFRINGENT SOLID FILM 0F EVAPORATED AND DEPOSITED ISOPHTHALIC ACID Filed March 24, 1.945

Evaporqhd and Dposihd lsophihalic Acid V/ t I! Glass FIG I FIG. 2

NVENTOR.

SEARCH ROOM S PATENT OFFICE UNITED STATE nmmmom SOLID mu EVAPO- RATIO AND DEPOSITS!) ISOII'I'HALIC ACID Elkan it. Bloat, Cambridge, Mala, aaaignor Polaroid Corporation, Cambridge, Mala, acorporatlen of Delaware Application March I. 1945. Serial No. 584,"!

6 Olaima. (Cl. 88-) 1 This invention relates to optical elements, and more particularly (3 the production of birefringent optical elements and material.

It is one object of the present invention to pro- 2 be insulated from base plate ii in any suitable manner. as for example by means of legs 2| of porcelain or other relatively non-heat-conductin: material. The charge II or material to be vide new and improved birefringent optical ele- 5 evaporated is placed within crucible 2| and heat ments of high birefringence, and particularly biapplied thereto by means of filaments 32 and 34 refrlngence in excess of 0.30. of any suitable metal and shape, a spiral shape Another object is to provide such highly bihaving been found desirable. Filaments 32 and refringent elements having the optical proper- 34 may be supported in any way as by means of ties of a basal section of a uniaxlal crystal. and th l ad ires th r t ot s wn. a d l t being optically isotropic for normally incident ll r p nts a r ively fin re r n. he light but birefringent for obliquely incident light. p rp 1' w i h will be de cribed herein f r- A further object is to provide optical elements In practicing the invention with the apparatus of the above type in the form of a solid illm comshown in Fig. 2, it is important that the vacuum prising isophthalic acid, and particularly to pro- Within bell J be as flh P s and be vide such birefringent optical elements by the maintained high throughout th v p i n high vacuum evaporation and deposition of isoproces Two factors contribute to this requir phthalic acid. ment. The first is that in the practice of the Still further objects and advantages will in pr se t vent on there is at least a sli t tendpart appear and in part b pointed out i th ency to decomposition of the charge at the temcourse of the following detailed description 0! D re 1' ev p r n. and this n y in' one or more embodiments of the invention. which creases with temperat re- At h s me ime. are given as nonlimiting examples, in connecany mp iti n results in ow rin f the tion with the accompanying drawings. in which: vacuum, which in turn raises the temp re Figure 1 is a sectional view illustrating dia- 1 r v ti n d hence n r s grammatically an embodiment of the invention; more pid p ition, and th e ffects are and cumulative unless any decomposition products Figure 2 is a sectional view ill strati di are removed as rapidly as they form; The other grammatically apparatus suitable for producing factor is related to the same problem and is that the embodiments of the invention shown in Fig. 1. org nic molecules en r ll nd t hold a s b- The embodiment of the invention shown in Fig. stantial amount of entrapped air r a which 1 comprises a supporting plate ll of glass r th will be released during evaporation and will suitable material having aillm l2 formed thereon thereby Similarly reduce t v m and by the evaporation and deposition of isophthalic courage decompo ition. A v m s hi h as acid under high vacuum. In accordance with 9! mercury 18 desirable and r P the present invention, it has been discovered that ferred re ul s it should not be permitted t dr p such a film will possess the optical properties of below 10- mm. of mercury. a basal section of an extremely highly birefrinn practicing the invention it is highly degent crystal, being optically isotropic for light sirable that heat be applied substantially uninormally incident thereon but exhibiting high f mly. a f r pr e r d results the entire birefringence for obliquely incident light up to a chill!!! should e hes-ted t0 the p r n point maximum of more than 0.30. Fig. 2 illustrates e e y va at n be inst appears that apparatus suitable for use in the preparation of Optimum results from the standpoint of ia device such as that shown in Fig, 1, formity and clarity are obtained when the charge Plate 20 in Fig. 2 corresponds to element II in vaporizes and dep 0!! Plate 19 in units as Fig. 1 and is illustrated as mounted on any suitsmall 88 p s le, i. e., molecules or group f a able supports 2! within bell jar 24, mounted on very small number of associated molecules. any suitable base plate 1!. It will be understood When the vaporization is not uniform there is a that any suitable means, not shown, may be proncy o the vapori material to ca r up vided for evacuating bell jar ll, and maintain-. with it particles of substantially greater than ing the desired high vacuum therein during the evaporation process. Crucible It may be formed from a non-heat-conducting material such as porcelain or a heat-conducting metal such as molecular size. and they in turn tend to cause loss of clarity and uniformity in the deposited coating. After evaporation begins, the control of heat is somewhat a matter of balance between stainless steel or nickel. in which case it may 66 maintaining the evaporation both rapid and uni.

the desired range may be aided in a variety of ways. For example, the use of a heat e'onducting' crucible and the two filaments 32 and 34 in Fig. In

2 aids in producing uniformly-rapid'heating} of' the charge. Screen 35 is of assistance in trapping particles of sizes such that they might afare quite 'hard but" relatively brittle. reason. "and-also. because they adhere thereto foot the quality of the depositedfi-lm' and i s,

therefore, preferably of relatively fine mes h.

stainless steel wire has been found satisfactory. Other factors 'afiecting uniformity are -the-dis-* tance and anguiiir relation between plate and crucible 26, which are dependent upon a number of variables. It is desirable to prevent the temperature of the plate from rising too high, ap-" proximately 60 to 65 C. being the preferred upper limit, and the plate should therefore not -be placed too near the crucible, from 3 to 4 inches being a preferred minimum distance. It is also preferred that the vaporized particles from the crucible deposit on the plate at as nearly normal incidence as possible, and the relative positionsof the plate and crucible should be determined accordingly. For example, if the plate is'p'osi tioned directly over the crucible as in Fig. '2, its lower surface should be' substantially parallel with the top of the crucible and at a distance therefrom which is deterr'nin'ed'to a consider able extent by their relative sizes- The rate" of deposition on any point on thepl'ate is approxi-' mately inversely proportional to the square bf the distance between said point and the crucible. It follows that the distance between plate and crucible should be correspondingly increased'for plates of substantially greater'area than the top of the crucible in order that the rate of deposition be as nearly uniform as possible over the entire surface of the plate. If there is a material departure from these conditions, the deposited film may be of non-uniform thickness or the optic axis therein may be non-uniformly disposed, or both of these effects may oc-" cur. The thickness of the deposited film depends upon the time of evaporation and the distance between the crucible and supporting plate, and it may be readily controlled either visually, if a transparent bell jar is used, or by means of any suitable measuring and control device such as a photoelectric measuring device. As' a s'pecific example, a film of'isophthalic acid having a thickness of the order of ten wavelengthsmay' be deposited in as little time as fifteen minutes from the time when heat is first applied to the charge.

When the foregoingconditi'on's are observed there will be 'formed on plate an optically clear and glassy appearing film possessing the' optical properties of a basal section of 'a" hi hly birefringent uniaxial crystal, -That-is to say, said film will be optically isotro'pic' for light normally incident thereon'b'ut will 'e'xhiliit high birefringence for obliquely incident light, the maximum value beingobserved in' a crosss'ectionf wherein the respective refractive indices have been measured as approximately 1.40am be- 4 tween 1.70 and 1.80. Examination by x-ray diffraction of the films of the invention reveals a pattern characteristic of a fibrous structure with the fiber axis perpendicular to the plane of the film, and the conclusion is that the film is composed cf a multiplicity ofmterocrystals which adhere to the plate and cohere together to form an integral film in which all said micro- .crystals are oriented parallel to a common direction which is perpendicular to the plane of said film. It is of-particular significance that said film is'uniaxial in;-lspite of the fact that isophthalic acid normally occurs in the form of biaxi'al ci-ys'talsg- The above described films of the invention For that quite strpngly, they are preferably retained in use on-the supporting plate whereon they are initially formed. It should be noted, however, that the :invehtion is' not limitedto sa'id films in combination-withaxsuppo'rt "or to the use of glass in said support} 'An'y' ofja v'ariety'of other: materials may 'be used, such for example as methyl 'metha'cr'ylat and-other similar organic resins. It is 'desirable' that the support used in the preparation of the 'filmbe'substantially rigid, since otherwise-it may bend under the conditions of evaporation. -A- convenient method of obtaining one of the films 'ofthe invention'free from any's'up'p'ort isio'diaposit said film on a support of watersoluble material, such for 'ex-- ample as rock salt,- and then to dissolve the latter away after the film has been formed. All

" such variations are to'be understoodas comingwithin the scope of 'the invention.

Since certain changesmay vbe made in the above product and different embodiments of the invention couldb made without departing from the scope thereof, it -is intended thatall matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed'is" l.' As a new product, a light transmitting, solid film comprisingisophthal-ic acid; the molecules of which form planar, optically anisotropic group-' ings' and" are 'oriented' in" substantial planar parallelism with'thesurfaces'of the film, whereby the film is optical-1y isotropic for light normally incident thereon and birefringent for light obliquely incident thereon, the optic axis of said film being disposed perpendicularly to the' film surface.

2. Asa new'prod'uct, an optical element comprising, in combination; means providing a supporting surface, and a substantially uniform solid film adhering'to'said surface and comprising isophthalic acid, the molecules of which are in the form of substantially planarf'optically anisotropic groupings and are oriented in substantial planar parallelism with the surfaces of the film, whereby the film is optically isotropic for light normally incident thereon and'bire'fringent for light obliquely incident thereon, the optic axisof said film bein'g'disposed' perpendicularly to the" film surface. V

i 3. As anew product, alighfitransmitting;solidflli'n comprising isophthalic'acid; the'mbmccies or" being disposed perpendicularly to the film surface, said film having an X-ray diffraction pattern characteristic of a fibrous structure with the fiber axis perpendicular to the plane of said film.

4. As a new product. an optical element comprising, in combination, means providing a supporting surface, and a substantially uniform solid film adhering to said surface and comprising isophthalic acid, the molecules of which are in the form of substantially planar, optically aniso tropic groupings and are oriented in substantial planar parallelism with the surfaces of the film, whereby the film is optically isotropic for light normally incident thereon and birefringent for light obliquely incident thereon, the optic axis of said film being disposed perpendicularly to the film surface, said film having an X-ray diffraction pattern characteristic of a fibrous structure with the fiber axis perpendicular to the plane of 5. In a process of forming a birefringent element, the stepsrq nmprising evaporating isophtha- SEARCH KUUM stantially uniformly throughout a mass of organic material comprising isophthalic acid until said mass begins to vaporize, and depositing said vaporized isophthalic acid upon the surface of a supporting plate positioned above said heated mass and spaced therefrom while maintaining the temperature of said supporting plate below 65 C., said plate being so positioned that the particles of said vaporized acid impinge thereon in a direction substantially normal to the surface of said plate, all of said steps being carried out in a vacuum.

ELKAN R. BLOUT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES, PATENTS Number Name Date 2,087,795 Chubb Jan. 20, 1937 2,270,323 Land et a1. Jan. 20, 1942 2,328,219 Land Aug. 31, 1943 OTHER REFERENCES positioned that the vaporized particles impinge thereon in a direction substantially normal to said surface, said steps being carried out in a McNally et al., article in Journal of Physical Chemistry, Vol. 34, January, June 1930, pages 165, 167, 168, 169. (Copy in Div.'7.)

Hackh's Chemical Dictionary, 3rd edition 1944, page 836, column 2. (Copy in Division '1.)

Hendricks in J. O. S. A., vol. 23, 1933, pages 299-307. (Copy in Division '7.) 

